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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/114206] recursive function call vs local
 variable addresses
Date: Mon, 04 Mar 2024 09:54:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114206

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-03-04

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
So we simplify this and RTL expand from

int f (int t, const int * a)
{
  const int b[4];
  _Bool _1;
  int _8;

  <bb 2> [local count: 1073741824]:
  if (t_3(D) =3D=3D 0)
    goto <bb 3>; [49.25%]
  else
    goto <bb 4>; [50.75%]

  <bb 3> [local count: 528857912]:

  <bb 4> [local count: 1073741824]:
  # a_11 =3D PHI <a_5(D)(2), &b(3)>
  _1 =3D &b =3D=3D a_11;
  _8 =3D (int) _1;
  b =3D{v} {CLOBBER(eos)};
  return _8;

We apply tail-recursion optimization here which coalescs both slots which
are used just by their address.  IIRC tail-recursion analysis uses alias
analysis to be able to handle some cases where TREE_ADDRESSABLE vars are
passed.

We miss considering variables live that are passed by reference (but otherw=
ise
are "unused").

As you needed to cut off quite some optimizations to early simplify the
b =3D=3D a compare this is a bit academic or needs much more obfuscation of=
 the
compare to actually matter.

But yes, it's a bug.=